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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan 3.0 MDJA Resurrection (or lets fix this junk)


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  #11  
Old 10-30-2017, 01:40:15 PM
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Billy J Shafer Billy J Shafer is offline
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

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Originally Posted by Ranchero50 View Post
Spent some more time browsing different part numbers and it looks like 147-0136 is correct for the MDJA. Funny how pricing is all over the place for them but at least they are available.

Looks like it's time to pull the rest of the cooling stuff off and see how hard it'll be getting parts for it. Once I know what I need I'll order everything at once. Hopefully that way I'll be able to avoid an 'Oh crap' moment. I'd love to deck the block but I don't think it'll fit in my CNC mill.
Very nice work. You might want to check about making Onan part 155-0782. Number 6 on exhaust manifold. Very common for them to break and can be hard to find
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  #12  
Old 10-30-2017, 04:26:53 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

That would be easy enough to make if I had dimensions. Mine being a single lung doesn't use the flange. Heck, comparing the service manual to what I have, mine is just cobbled together pipe fittings with an honest to goodness 5/8" opening in the gasket material at the head port end. Jeesh, I can only imagine how poorly it ran.

I'm pretty sure I have some DOM SS tubing floating around from my dairy mechanic days.
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Old 10-30-2017, 09:05:23 PM
JohnnyC JohnnyC is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

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So, lets move this project forward a little. Needed the flange that holds the injector in the bore. Still seems such an odd way of doing things to just have the injector held in the bore by two 5/16" bolts but I guess it works.

Made a sketch with dimensions and then fired up my CAD program to draw the part (blue line) and sort out the tool path for a 1/4" carbide end mill (red line). Then because my Windows 10 shop puter still won't talk to my dos 6.22 mill I needed to transcribe the G code manually...


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I think I'd like to play in your shop for about 30 years or more.

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  #14  
Old 11-01-2017, 12:38:59 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

$7 and a trip to NAPA later and the fuel system is back together. I like the plastic coated lines as they don't rust like the regular stuff does.



Mocked up the valve train and used a dowel in the injector bore to set TDC on the pulley. Been slow going but at least we are moving forward.
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Old 11-13-2017, 11:00:29 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

Spent some more time with the Onan this week. Set the valve and buttoned up the top end. Mocked up a fuel and battery to see if I can get the fuel system to purge and maybe fire off. Nothing past the exhaust port so it's going to be LOUD if it takes off and actually runs.

Battery is weak but it did turn over enough to get fuel up to the injector. The whole thing is sitting on a sheet metal rolling cart and hops around a good bit just rolling over.
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  #16  
Old 11-14-2017, 11:38:21 AM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

No joy yet. Good bit of grey smoke so it's injecting but not firing off yet. Heater coil works well and I found a weak connection for the battery (smoking hot) thanks to a rusty bolt. Shut off solenoid is working as well. Waiting on the battery to come up before round 3 (or so).

Rummaging around my left over parts I also just realized I didn't hook the glow plug up. Ohm'd it out and it's good so half terrified, half excited...
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Old 11-14-2017, 01:26:15 PM
Ranchero50 Ranchero50 is offline
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So, got smoke, lots of smoke. Got fire and it'll eventually smooth out and run after a couple minutes of holding the start and glow plug buttons. Need to put a good battery on it as the one I'm using is pretty mush shot (no reserve). Leary to run it too long without the cooling system fully hooked up. Coolant in the engine is turning muddy, so I'm not too sure about the head gasket vs. oil contamination during the work. I'm sure it'll let me know. Oil looks and smells good, maybe a little diesel but that's expected. I'll change it once I get it running reliably.

It'll stay running if I hold the decomp relay shut. Getting 4vac 60hz out of the mains. Been trying to clean up the slip rings with a Scotchbrite and plastic handles while bumping it over and they are getting there. I assume once it generates it'll hold the relay shut. Good oil pressure and the over temp switch was bypassed (broken).





It's really not that loud once it's running which surprised me. Little bouncy on the cart but not too bad. Guess the big old hunk of iron keeps it civilized. Overall pretty happy.
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  #18  
Old 11-14-2017, 04:38:38 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

OK, struggling here. I can't get the decomp relay to hold in. I'm only getting about .5vdc at the positive side of the coil. The ground ckt with oil pressure switch is good. Checked and I'm getting .65vdc to the field at the flicker points terminal. The flicker points are changing states and the gap is correct (.025"). It's been forever since I troubleshot an AC genset. Looking for a quick push in the right direction.

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Old 11-15-2017, 01:38:36 AM
Dennis19508 Dennis19508 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

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Originally Posted by Ranchero50 View Post
OK, struggling here. I can't get the decomp relay to hold in. I'm only getting about .5vdc at the positive side of the coil. The ground ckt with oil pressure switch is good. Checked and I'm getting .65vdc to the field at the flicker points terminal. The flicker points are changing states and the gap is correct (.025"). It's been forever since I troubleshot an AC genset. Looking for a quick push in the right direction.
Glad you have it running. Making good progress.

Looking at the schematic, first thing to note is that the start stop switch is labeled backwards. Start connects the center wiper to the LEFT contact which connects ground to the bottom of the start solenoid coil, causing its contacts to close.

So then B+ comes out of the S1 contact of the start solenoid, thru the 6 ohm resistor at the upper left of the schematic then to the "gen" terminal of the reverse current relay. Then thru the 15 ohm resistor to pin 3 of the decompression solenoid relay. So make sure you have close to 12 V at pin 3 during cranking. If not check both resistors.

Next thing that needs to happen is for oil pressure switch to close which will ground pin 4 of the decompression solenoid relay causing its contacts to close.

Pin 1 of the decompression solenoid relay should always have B+ provided thru the amp-meter. If it doesn't have B+ there check the amp-meter connections. Now assuming everything is working up to this point B+ should appear on pin 2 of the decompression solenoid relay and energize the decompression solenoid.

So at this point the question is does the decompression solenoid energize during cranking? If it does, but drops out after you release the start switch, then look at the reverse current relay. If it doesn't check everything noted above.

If the reverse current relay is an open type relay you might be able to see it move during cranking. It should stay closed once the generator is running and making DC. If open frame style you can push it closed with your finger before releasing the start button. If it then keeps running then you know the problem area. During normal operation I think it is held shut by the 12 volt battery charging current from the generator S1. Since it cranks, this should be working and producing output, but I've never had to troubleshoot the DC part of the JA genhead so I might be wrong.

Good luck and happy hunting. Hope you bag the problem.
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  #20  
Old 11-15-2017, 01:11:01 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

Thanks for the ideas. Answers in red.

Quote:
Originally Posted by Dennis19508 View Post
Looking at the schematic, first thing to note is that the start stop switch is labeled backwards. Start connects the center wiper to the LEFT contact which connects ground to the bottom of the start solenoid coil, causing its contacts to close.

So then B+ comes out of the S1 contact of the start solenoid, thru the 6 ohm resistor at the upper left of the schematic then to the "gen" terminal of the reverse current relay. Then thru the 15 ohm resistor to pin 3 of the decompression solenoid relay. So make sure you have close to 12 V at pin 3 during cranking. If not check both resistors. All good.

Next thing that needs to happen is for oil pressure switch to close which will ground pin 4 of the decompression solenoid relay causing its contacts to close. They do. Pin 4 grounds when running.

Pin 1 of the decompression solenoid relay should always have B+ provided thru the amp-meter. If it doesn't have B+ there check the amp-meter connections. Now assuming everything is working up to this point B+ should appear on pin 2 of the decompression solenoid relay and energize the decompression solenoid. Pin one is what drops out when I let off the 'start' button.

So at this point the question is does the decompression solenoid energize during cranking? If it does, but drops out after you release the start switch, then look at the reverse current relay. If it doesn't check everything noted above. Yes, Decomp relay pulls in allowing it to run while the start button is held. Drops out if left off. It's open so I can hold the reed closed and keep it running (by hand)

If the reverse current relay is an open type relay you might be able to see it move during cranking. It should stay closed once the generator is running and making DC. If open frame style you can push it closed with your finger before releasing the start button. If it then keeps running then you know the problem area. During normal operation I think it is held shut by the 12 volt battery charging current from the generator S1. Since it cranks, this should be working and producing output, but I've never had to troubleshoot the DC part of the JA genhead so I might be wrong. .5vdc on the S1 while running, battery volts while 'start' button is held.

Good luck and happy hunting. Hope you bag the problem.
So I guess I have a DC gen problem. Looking at the DJA service manual (967-0511) it shows the shunt and field winding. Shunt should be good to go since it's getting hit with battery volts to crank. I wouldn't think it would be a slip ring problem since it cranks too.
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